In a typical separate type air-conditioner, an outdoor unit is connected with an indoor unit and has an air conditioning capacity appropriate for the rooms in which the indoor unit is installed.
This implies that the same number of indoor and outdoor units must be set up if all the rooms are air-conditioned with this type of air conditioning system, which is very costly and requires a great amount of installation work. As a solution to this problem, a multiplex air-conditioning system has been proposed, in which a multiplicity of indoor units are connected with a single outdoor unit having a large capacity.
If, however, each of such indoor units are connected independently of each other with the outdoor unit via a pair of tubes for the circulation of the refrigerant, the total length of the refrigerant tubes will be disadvantageously large, since each of the indoor units must be connected with the outdoor unit by an independent pair of refrigerant tubes. As a consequence of such tubing, an appreciable pressure drop will result in the tubes and hence the outdoor unit must have a large cooling capacity to compensate the pressure drop. In addition, the outdoor unit must have a complex tubing unit for connecting many refrigerant tubes. This also causes a problem of complex tubing and an extra cost.
Recent development in architectural technology has enabled construction of a building which is thermally well insulated, with rooms having good insulation. This is advantageous for a contemporary house for a family having individual small rooms rather than having a fewer but larger traditional rooms. In this case it is not economical both from points of running cost and construction cost to set up an independent air conditioning unit for each room, since individual rooms do not need a large air conditioning capacity. It would be advantageous to install a multiplex air conditioning system in air-conditioning a group of such well insulated small rooms if the air-conditioning system can be so controlled as to provide refrigerant to these indoor units as needed, because most of these indoor units require only a small amount of refrigerant. Unfortunately, however, conventional multiplex air-conditioning systems are normally designed to distribute refrigerant evenly to all indoor units connected, and are not capable of controlling the flows to the individual indoor units, so that a large indoor unit, if present, cannot obtain sufficient refrigerant.